High Speed Downlink Packet Access (HSDPA) is one of the key features of the third generation wireless communication standard for Wide Band Code Division Multiple Access (W-CDMA). W-CDMA is proposed to support multimedia services in the downlink direction. HSDPA brings high speed data delivery to 3G terminals, ensuring that users requiring effective multimedia capabilities benefit from data rates previously unavailable due to limitations in the radio access network between the user terminal and the base station. HSDPA will provide very high data rates in the downlink direction. In order to achieve these high data rates, several key technologies, such as Adaptive Modulation and Coding (AMC) and Hybrid-ARQ (Automatic Repeat Request) have been considered and adopted in the relevant telecommunication standards. Furthermore, fast signalling techniques have also been used for HSDPA to enhance the AMC link adaptive technique. For HSDPA, the fast signalling technique is applied using the High Speed Shared Control Channels (HS-SCCH) which carries control information for receiving and decoding the associated data channels (HS-DSCH) which is transmitted two slots after the control channel.
In order to ensure the quality of the control information received by a mobile terminal, a high spreading factor channel is used. A spreading factor of 128 is defined for the HS-SCCH. In order to reduce the number of information bits transmitted, certain types of information are encoded with only indicator information being sent to the mobile terminal. This information includes associated HS-PDSCH channelisation codes, transport block size, redundancy version parameter, constellation version parameter, and modulation scheme. This encoded information is sent on a shared control channel which is always two slots in advance of the associated physical data channel (HS-PDSCH). Upon detection of the share control channel intended for it, the mobile terminal proceeds to decode and retrieve control parameters sent on the HS-SCCH to receive and decode the associated HS-PDSCHs which shall arrive in later slots. As part of reducing the number bits transmitted on the HS-SCCH, the channelisation codes used for reception of the associated HS-PDSCHs are encoded so as to be “self-decodable” at the mobile terminal.
For HSDPA, the W-CDMA network needs to signal to the mobile terminal exactly how many channelisation codes have been allocated and at which offset the set of codes begin. In HSDPA, the number of channelisation codes and the code offset of the associated high speed data channel (HS-PDSCH) are encoded to form 7 bits of channelisation code set information xccs,1,xccs,2,xccs,3,xccs,4,xccs,5,xccs,6,xccs,7. The first 3 bits of the channelisation code set information represent the number of channelisation codes, whereas the last 4 bits represent the code offset.
During transmission to the mobile terminal, the number P of channelisation codes and the code offset O are encoded as follows:xccs,1,xccs,2,xccs,3=min(P−1, 15−P).xccs,4,xccs,5,xcss,6,xccs,7=|O−1−└P/8┘*15|
Currently, there is no direct and generic solution to solve the above-referenced equations for P and O from the 7-bit channelisation code set information. A traditional solution to decode P and O from the channelisation code set information is to construct a lookup table which contains all possible combinations of the 7 bits forming the channelisation code set information. However, this solution requires a high level of processing power to search, compare and select a correct value for P and O. Moreover, this solution requires additional memory to store a pre-generated lookup table.
It would be advantageous to provide a method for decoding channelisation code set information to determine the number of channelisation codes and the code set information in a spread spectrum data transmission that ameliorates or overcomes the disadvantages of known channelisation code set information decoding methods.
It would also be desirable to provide a method of decoding channelisation code set information to determine the number of channelisation codes and the code offset in a spread spectrum data transmission that minimised the processing power and/or memory requirements of a mobile terminal in which the decoding was performed.